1. Field of the Invention
The present invention relates to an improvement of an optical waveguide switch. More particularly, the present invention relates to a reflection type optical switch in which switching occurs when the refractive index is changed which leads to changes is reflectance due to carrier injection at the branch of the optical path, characterized by the branch of the optical path having a current confinement structure so that the extinction ratio is improved. The present invention also relates to an optical directional coupler switch utilizing the optical coupling between two waveguides having improved modulation efficiency low voltage operation.
2. Description of the Prior Art
As the optical communications and optical data processing become more complex, sophisticated, and diversfied, there has arisen a demand for optical components which are small, reliable, and have fast responses. For optical switches which are an element of an optical integrated circuit, studies are being made to develop a new optical switch which has a low loss, low drive voltage, fast response, and high extinction ratio. (See H. Kawaguchi, "GaAs RIB-WAVEGUIDE DIRECTIONAL COUPLER SWITCH WITH SCHOTTKY BARRIERS", Electronics Lett., 14 No. 13,387 (1978).) An example of a prior art optical switch is described with reference to FIG. 10, which is a perspective view of a conventional optical directional coupler switch having a pn junction. This switch is made up of an n.sup.+ -GaAs substrate (carrier concentration: 10.sup.18 cm.sup.-3) (101), an n-Ga.sub.1-x Al.sub.x As buffer layer (102), n.sup.- -GaAs optical waveguide layer (carrier concentration: lower than 10.sup.15 cm.sup.-3) (103), a p-Ga.sub.1-y Al.sub.y As buffer layer (105), a p.sup.+ -GaAs cap layer (carrier concentration: 10.sup.18 cm.sup.3) (106), and AuGeNi ohmic electrodes (107, 108). The first three layers (101, 102, 103) form a ridge type optical waveguide. FIG. 11(a) schematically shows the distribution of refractive index in the above-mentioned optical switch. The thicknesses of p.sup.+ -GaAs cap layer (106), p-Ga.sub.1-6 Al.sub.y As buffer layer (105), n.sup.- -GaAs optical waveguide ridge (104), n.sup.- -GaAs optical waveguide layer (103), and n-Ga.sub.1-x Al.sub.x As buffer layer (102) are represented by t.sub.1, t.sub.2, t.sub.5, t.sub.6, and t.sub.4, respectively. n.sup.+ -GaAs substrate (101) and p.sup.+ -GaAs cap layer (106) have a refractive index of N.sub.o ; n.sup.- -GaAs optical waveguide ridge (104) and optical waveguide layer (103) have a refractive index of N.sub.1 ; and p-Ga.sub.1-y Al.sub.y As buffer layer (105) and n-Ga.sub.1-x Al.sub.x As buffer layer (102) have a refractive index of N.sub.2. The two buffer layers (105 and 102) have the same molar fraction of Al and therefore x=y.
The width and thickness (t.sub.3) of the ridge of the optical waveguide, and x and y are properly selected so that the light propagates in the optical waveguide in the fundamental mode (single mode) alone. In other words, the distribution of the intensities of the optical electric field varies according to the refractive index and thickness of the individual layers. For example, if the thickness of the optical waveguide ridge (104) is increased, the light converges into the optical waveguide layer (103).
FIG. 11(b) schematically shows the distribution of the intensities of the electric field (the distribution of the fundamental mode) of the light that propagates in the optical waveguide of the optical switch shown in FIG. 10. It also schematically shows the distribution of the electric field of the depletion region in the pn junction. The distribution is plotted against the thickness in the direction from p.sup.+ -GaAs cap layer (106) to n.sup.+ -GaAs substrate (101). The electric field of the depletion region takes the maximum value (.epsilon..sub.m) at the pn junction interface and the optical electric field takes the maximum value somewhere in the n.sup.- -GaAs optical waveguide (near n-Ga.sub.1-x Al.sub.x As buffer layer (102) rather than the center).
The switching of a light propagation is accomplished by applying a reverse bias voltage to the pn junction from the power source (109), so that the refractive index in the depletion region is increased and a change takes place in the condition of the coupling between the two parallel optical waveguides. In order for the light switching (or modulation) to be performed effectively in accordance with the applied voltage, it is necessary that the intensity of the electric field of the depletion region and the intensity of the optical electric field overlap with each other at their maximum values. However, this overlap is not seen in FIG. 11(b). In other words, the optical switch of conventional structure as shown in FIG. 10 does not perform effective switching in accordance with the applied voltage.
A reflection type optical switch that utilizes carrier injection, is described in "Waveguided optical switch in InGaAs/InP using free-carrier plasma dispersion", Mikami et al., Electronics Lett., Vol. 20, No. 6, 15th March 1984, p. 228-229. In the optical switch of this type, the injected carriers spread out, causing the refractive index to change over a broad region, and the boundary between the region in which the refractive index is changed and the region in which the refractive index remains unchanged also spreads out. Therefore, it is necessary to inject a large amount of current in order to perform the switching operation. In addition, the optical switch of this type does not have a good extinction ratio because the light leaks into the region in which the refractive index has been changed. If the carrier injection region is lengthened to improve the extinction ratio, the absorption loss increases and the device becomes large, which is a hindrance to practical use.